1. Field to which the Invention belongs
The present invention relates to an apparatus for producing low-carbon cold steel sheet having cold workability, particularly press forming quality with use of a continuous annealing system.
The term "a low-carbon steel sheet" used hereinafter designates a steel sheet which is produced by the process of cold rolling and annealing and is for such applications as pressed automobile body parts, rather than an insufficiently annealed steel sheet which is for zinc-plating or tin-plating not subjected to drawing.
2. Prior art
Thin steel sheet for cold working, particularly press forming for use in automobile parts must have among its properties good ductility as well as good drawability and stretchability. To obtain these properties, it is required that the grain size be large enough to prevent the condition known as surface orange peel due to press forming, that the content of dissolved carbon and nitrogen be sufficiently low, that the yield point be low and the elongation be large. To meet these requirements, most of the thin cold rolled steel sheets for such applications have been conventionally produced by box annealing.
However, box annealing requires a longer treatment time (normally more than 60 hours) and thus is undesirable from the standpoint of efficiency. A method for reducing the treatment time recently has been proposed as disclosed in Japanese Patent Publication Sho 43-5995. This method still requires more than 30 hours for the treatment and thus is completely non-competitive with continuous annealing.
Several patents have been published regarding continuous annealing of cold rolled steel sheet for tin-plating. Among these patents, U.S. Pat. No. 2,832,711 is noteworthy. Although this patent discloses relatively soft thin steel sheet treated by a continuous annealing, it discloses or suggests nothing with respect to low-carbon steel sheet having excellent workability which is for pressed autobody parts. Commercial production of such steel sheet has never been carried out.
Reasons why low-carbon cold rolled steel sheet for applications such as pressed automobile body parts has never been produced by means of a continuous annealing are as follows.
It has been generally believed that low yield point, large elongation and Lank-Ford value (r-value) which can be obtained by box annealing can not be obtained unless the speed of the sheet pass is greatly reduced to provide an overaging time for effecting full precipitation of dissolved carbon, or unless the sheet, after continuous annealing for recrystallization is coiled and given enough aging time while the sheet is in a coiled state. For the former case, a treatment furnace for the overaging must be extremely long and thus is not practical for commercial production. In the latter case, the advantages of a continuous treatment as a whole can not be obtained although the annealing is effected continuously.
Further, from the standpoint of equipment, in order to effect recrystallization annealing - overaging - temper rolling - leveling - and recoiling in a continuous manner, it is necessary to rapidly cool the sheet after the overaging to near room temperatures for the subsequent temper rolling, and it is also necessary to perform very quickly the replacement of rolls of the temper rolling mill without interruption of the continuous treatment of the sheet.
No appropriate means has been provided for meeting the above requirements.